The present invention pertains to (i) a method of making an article that has a polymer coating disposed on a microstructured substrate, and to (ii) an article that possesses a microstructured surface and that has a profile-preserving polymer coating disposed on the surface.
Various techniques are known for coating substrates with thin layers of polymeric materials. In general, the known techniques can be predominantly divided into three groups, (1) liquid coating methods, (2) gas-phase coating methods, and (3) monomer vapor coating methods. As discussed below, some of these methods have been used to coat articles that have very small surface feature profiles.
Liquid Coating Methods
Liquid coating methods generally involve applying a solution or dispersion of a polymer onto a substrate or involve applying a liquid reactive material onto the substrate. Polymer or pre-polymer application is generally followed by evaporating the solvent (in the case of materials applied from a solution or dispersion) and/or hardening or curing to form a polymer coating. Liquid coating methods include the techniques commonly known as knife, bar, slot, slide, die, roll, or gravure coating. Coating quality generally depends on mixture uniformity, the quality of the deposited liquid layer, and the process used to dry or cure the liquid layer. If a solvent is used, it can be evaporated from the mixture to form a solid coating. The evaporation step, however, commonly requires significant energy and process time to ensure that the solvent is disposed of in an environmentally-sound manner. During the evaporation step, localized factorsxe2x80x94which include viscosity, surface tension, compositional uniformity, and diffusion coefficientsxe2x80x94can affect the quality of the final polymer coating.
Liquid coating techniques can be used to coat materials onto substrates that have small surface feature profiles. For example, U.S. Pat. No. 5,812,317 discloses applying a solution of prepolymer components and a silane coupling agent onto the protruding portions of partially embedded microspheres. And U.S. Pat. No. 4,648,932 discloses extruding a liquid resin onto partially embedded microspheres. As another example, U.S. Pat. No. 5,674,592 discloses forming a self-assembled-monolayer coating of octadecyl mercaptan and a partially fluorinated mercaptan (namely, C8F17(CH2)11SH) from a solvent onto a surface that has small surface feature profiles.
Gas-phase Coating Methods
Gas-phase coating techniques generally include the methods commonly known as physical vapor deposition (PVD), chemical vapor deposition (CVD), and plasma deposition. These techniques commonly involve generating a gas-phase coating material that condenses onto or reacts with a substrate surface. The methods are typically suitable for coating films, foils, and papers in roll form, as well as coating three-dimensional objects. Various gas-phase deposition methods are described in xe2x80x9cThin Films: Film Formation Techniques,xe2x80x9d Encyclopedia of Chemical Technology, 4th ed., vol. 23 (New York, 1997), pp. 1040-76.
PVD is a vacuum process where the coating material is vaporized by evaporation, by sublimation, or by bombardment with energetic ions from a plasma (sputtering). The vaporized material condenses to form a solid film on the substrate. The deposited material, however, is generally metallic or ceramic in nature (see Encyclopedia of Chemical Technology as cited above). U.S. Pat. No. 5,342,477 discloses using a PVD process to deposit a metal on a substrate that has small surface feature profiles. A PVD process has also been used to sublimate and deposit organic materials such as perylene dye molecules onto substrates that have small surface features, as disclosed in U.S. Pat. No. 5,879,828.
CVD processes involve reacting two or more gas-phase species (precursors) to form solid metallic and/or ceramic coatings on a surface (see Encyclopedia of Chemical Technology as cited above). In a high-temperature CVD method, the reactions occur on surfaces that can be heated at 300xc2x0 C. to 1000xc2x0 C. or more, and thus the substrates are limited to materials that can withstand relatively high temperatures. In a plasma-enhanced CVD method, the reactions are activated by a plasma, and therefore the substrate temperature can be significantly lower. CVD processing can be used to form inorganic coatings on structured surfaces. For example, U.S. Pat. No. 5,559,634 teaches the use of CVD processing to form thin, transparent coatings of ceramic materials on structured surfaces for optical applications.
Plasma deposition, also known as plasma polymerization, is analogous to plasma-enhanced CVD, except that the precursor materials and the deposited coatings are typically organic in nature. The plasma significantly breaks up the precursor molecules into a distribution of molecular fragments and atoms that randomly recombine on a surface to generate a solid coating (see Encyclopedia of Chemical Technology as cited above). A characteristic of a plasma-deposited coating is the presence of a wide range of functional groups, including many types of functional groups not contained in the precursor molecules. Plasma-deposited coatings generally lack the repeat-unit structure of conventional polymers, and they generally do not resemble linear, branched, or conventional crosslinked polymers and copolymers. Plasma deposition techniques can be used to coat structured surfaces. For example, U.S. Pat. No. 5,116,460 teaches the use of plasma deposition to form coatings of plasma-polymerized fluorocarbon gases onto etched silicon dioxide surfaces during semiconductor device fabrication.
Monomer Vapor Coating Methods
Monomer vapor coating methods may be described as a hybrid of the liquid and gas phase coating methods. Monomer vapor coating methods generally involve condensing a liquid coating out of a gas-phase and subsequently solidifying or curing it on the substrate. The liquid coating generally can be deposited with high uniformity and can be quickly polymerized to form a high quality solid coating. The coating material is often comprised of radiation-curable monomers. Electron-beam or ultraviolet irradiation is frequently used in the curing (see, for example, U.S. Pat. No. 5,395,644). The liquid nature of the initial deposit makes monomer vapor coatings generally smoother than the substrate. These coatings therefore can be used as a smoothing layer to reduce the roughness of a substrate (see, for example, J. D. Affinito et al., xe2x80x9cPolymer/Polymer, Polymer/Oxide, and Polymer/Metal Vacuum Deposited Interference Filtersxe2x80x9d, Proceedings of the 10th International Conference on Vacuum Web Coating, pp. 207-20 (1996)).
As described above, current technology allows coatings to be produced which have metal, ceramic, organic molecule, or plasma-polymerized layers. While the known technology enables certain coatings to be applied onto certain substrates, the methods are generally limited in the scope of materials that can be deposited and in the controllability of the chemical composition of the coatings. Indeed, these methods are generally not known to be suitable for producing cured polymeric coatings on microstructured surfaces that have controlled chemistry and/or that preserve the microstructured profile. While the techniques described above are generally suitable for coating flat surfaces, or substrates having macroscopic contours, they are not particularly suited for coating substrates that have microstructured profiles because of their inability to maintain the physical microstructure.
Some substrates have a specific surface microstructure rather than a smooth, flat surface. Microstructured surfaces are commonly employed to provide certain useful properties to the substrate, such as optical, mechanical, physical, biological, or electrical properties. In many situations, it is desirable to coat the microstructured surface to modify the substrate properties while retaining the benefits of the underlying microstructured surface profile. Such coatings therefore are generally thin relative to the characteristic microstructured surface dimensions. Of the thin-film coating methods described above, few are capable of depositing uniform thin coatings onto microstructured surfaces in a manner that retains the underlying physical microstructured surface profile.
The present invention provides a new method of coating a microstructured surface with a polymer. The method comprises the steps: (a) condensing a vaporized liquid composition containing a monomer or pre-polymer onto a microstructured surface to form a curable precursor coating; and (b) curing the precursor coating on the microstructured surface.
This method differs from known methods of coating microstructured surfaces in that a vaporized liquid composition is condensed onto a microstructured surface to provide a curable coating that is cured on the microstructured surface. The method is capable of producing polymeric coatings that preserve the microstructured profile of the underlying substrate. Known methods of coating microstructured articles involved coating reactive liquid materials from a solution or dispersion, sublimating whole molecules, or depositing atoms and/or molecular fragments. These known techniques were not known to provide polymer coatings that preserved the profile of the underlying microstructured substrate and that had controlled chemical composition.
A product that can be produced from the inventive method thus is different from known microstructured articles. The present invention accordingly also provides an article that has a microstructured surface that has a profile-preserving polymer coating disposed on the microstructured surface. The polymer coating not only preserves the profile of the microstructured surface, but it also controls the chemical composition. Thus, the polymer coating also has a controlled chemical composition. In an alternative embodiment, a microstructured substrate can be coated such that it has multiple profile-preserving coatings to form a multilayer coating.
The present invention provides the ability to coat a wide range of polymer-forming materials on microstructured surfaces to yield coatings that maintain the microstructured profile and that have controlled chemical compositions. This in turn allows the surface properties of the microstructured substrate to be changed (i.e., be replaced or enhanced with the surface properties of the coating) without adversely affecting the structural properties of the original surface. Additionally, multiple profile-preserving coatings of the same or different materials can be deposited to further affect one or more surface properties, such as optical properties, electrical properties, release properties, biological properties, and other such properties, without adversely affecting the profile of the microstructured substrate.
Desired fabrication techniques as well as end use applications can limit the range of materials that can be used to form microstructured substrates. Thus, while microstructured articles can be readily made to yield desired microstructural properties, the surface of the microstructured article might have undesirable (or less than optimal) physical, chemical, electrical, optical, biological properties, or other surface properties.
The present invention can provide microstructured substrates with a wide variety of surface properties that might not otherwise be attainable by conventional means while still maintaining the microstructured profile of the substrate. By depositing a profile-preserving polymer coating on a microstructured surface according to the present invention, the structural properties of the microstructured substrate can be maintained while changing or enhancing one or more of various physical, optical, or chemical properties of the microstructured surface. The profile-preserving polymer coatings of the present invention also have a controlled chemical composition, which helps achieve and maintain surface property uniformity across desired substrate areas.
The above and other advantages of the invention are more fully shown and described in the drawings and detailed description of this invention. It is to be understood, however, that the description and drawings are for illustrative purposes and should not be read in a manner that would unduly limit the scope of the invention.
As used in this document, the following terms have the following definitions:
xe2x80x9cCondensingxe2x80x9d means collecting gas-phase material on a surface so that the material resides in a liquid or solid state on the surface.
xe2x80x9cControlled chemical compositionxe2x80x9d defines a polymer coating that has a predetermined local chemical composition characterized by monomer units joined, for example, by addition, condensation, and/or ring-opening reactions, and whose chemical composition is predetermined over lateral distances equaling at least several multiples of the average coating thickness, where the following meanings are ascribed: xe2x80x9cpredeterminedxe2x80x9d means capable of being known before making the coating; xe2x80x9clateralxe2x80x9d is defined by all directions perpendicular to the thickness direction; and the xe2x80x9cthickness directionxe2x80x9d is defined for any given position on the coating as the direction perpendicular to the underlying surface profile at that position.
xe2x80x9cCuringxe2x80x9d means a process of inducing the linking of monomer and/or oligomer units to form a polymer.
xe2x80x9cFeaturexe2x80x9d, when used to describe a surface, means a structure such as a post, rib, peak, portion of a microsphere, or other such protuberance that rises above adjacent portions of the surface, or a structure such as a groove, channel, valley, well, notch, hole, or other such indentation that dips below adjacent portions of the surface. The xe2x80x9csizexe2x80x9d or xe2x80x9cdimensionxe2x80x9d of a feature includes its characteristic width, depth, height, or length. Of the various dimensions in a microstructured surface profile, the xe2x80x9csmallest characteristic dimension of interestxe2x80x9d indicates the smallest dimension of the microstructured profile that is to be preserved by a profile-preserving polymer coating according to the present invention.
xe2x80x9cMicrostructured substratexe2x80x9d means a substrate that has at least one surface that has an intended plurality of features that define a profile characterized by local minima and maxima, the separation between neighboring local minima and/or maxima being about 1 micrometer (xcexcm) to about 1000 xcexcm. The separation between two points on the surface refers to the distance between the points in any direction of interest.
xe2x80x9cMonomerxe2x80x9d refers to a single, one unit molecule that is capable of combining with itself or with other monomers or oligomers to form other oligomers or polymers.
xe2x80x9cOligomerxe2x80x9d refers to a compound that is a combination of 2 or more monomers, but that might not yet be large enough to qualify as a polymer.
xe2x80x9cPolymerxe2x80x9d refers to an organic molecule that has multiple carbon-containing monomer and/or oligomer units that are regularly or irregularly arranged. Polymer coatings made according to the present invention are prepared by linking together condensed monomers and/or oligomers so that at least a portion of the polymer coating""s chemical structure has repeating units.
xe2x80x9cPre-polymerxe2x80x9d includes monomers, oligomers, and mixtures or combinations thereof that are capable of being physically condensed on a surface and linked to form a polymer coating.
xe2x80x9cPrecursor coatingxe2x80x9d means a curable coating that, when cured, becomes a polymer coating.
xe2x80x9cProfile-preserving coatingxe2x80x9d means a coating on a surface, where the outer profile of the coating substantially matches the profile of the underlying surface for feature dimensions greater than about 0.5 xcexcm and smoothes the profile of the underlying surface for feature dimensions less than about 0.5 xcexcm; where xe2x80x9csubstantially matchingxe2x80x9d includes surface profile deviations of no more than about 15%, that is, each dimension (such as length, width, and height) of the surface profile after coating deviates by no more than about 15% of the corresponding dimension before coating. For profile-preserving coatings that include multiple layer stacks, at least one layer of the multiple layer stack is a profile-preserving coating.
xe2x80x9cVaporxe2x80x9d, when used to modify the terms xe2x80x9cmonomerxe2x80x9d, xe2x80x9coligomerxe2x80x9d, or xe2x80x9cpre-polymerxe2x80x9d, refers to monomer, oligomer, or pre-polymer molecules in the gas phase.